The present invention generally relates to a preprocessor for preprocessing a plurality of electromagnetic signals received by an adaptive antenna array. More specifically, the present invention relates to a preprocessor that frequency smears and averages the electromagnetic signals such that an output signal of the preprocessor contains sufficient information to enable an adaptive weight calculator to calculate an accurate weighting coefficient for the electromagnetic signal, which weighting coefficient is then used to eliminate interference contained in the electromagnetic signal. The present invention is also applicable to adaptive processing systems in general, including adaptive filters.
Adaptive antenna array systems adaptively reconfigure the signals received by an array of antenna elements generally for the purpose of improving the reception of the received signal in the presence of jamming, noise, and other interference. An adaptive array provides this capability by modifying the receive gain pattern of an antenna array. For example, one can adjust the receive antenna pattern to maximize the receive gain in the angular direction of a desired signal source while simultaneously minimizing the gain in the direction of an interference source. The gain pattern is modified by adjusting the adaptive array weighting coefficients; in the simplest case, there is one coefficient for each antenna element of the antenna array. If the angular locations of the signal/interference sources are known, the value of the weighting coefficients that achieve the desired gain pattern can be calculated without further information (assuming the antenna array is well calibrated). However, if their locations are unknown, as is often the case, the weighting coefficients can only be determined from information extracted from the signals (including interference) received by the array. The latter approach, which describes the adaptive processing concept, has proven quite effective and, as a result, has found use in many military and commercial radar, communication, and navigation systems.
An adaptive array antenna system (which may be more generally referred to as a spatial filter, or smart antenna) generally includes a plurality of antenna elements for receiving electromagnetic signals. The output of each antenna element is generally provided to an adaptive weight calculator that is programmed to calculate a weighting coefficient, which is then applied to the electromagnetic signal received by the antenna element in order to create the desired array pattern. For example, in military applications the adaptive weight calculator may be xe2x80x9clookingxe2x80x9d to eliminate jamming signals. If a jamming signal is detected, the adaptive weight calculator would eliminate the jamming signal or reduce its impact by, for example, substantially reducing the gain on such signal.
In order to perform the adaptive weight calculations in an accurate and timely manner, information must be extracted from the output of each antenna element. In cases where the antenna array consists of hundreds or even thousands of antenna elements, the processing power required to perform the weight calculations can be significant.
In an effort to reduce the required processing power, xe2x80x9cshortcutsxe2x80x9d have been developed. For example, the number of samples provided the adaptive weight calculator for use in the weight calculation may be reduced by a predetermined factor, for example, a factor of 10, by only using every 10th sample in the weight calculation (instead of using every sample). Such an approach is referred to as sparse sampling. This is appropriate in situations where the interference waveform and location do not change much (e.g., a tone jammer or interference signal emanating from a stationary or slowly moving transmitter), where the reduced number of samples available for use by the adaptive weight calculator is often not critical. In this case, the use of fewer samples in the weight calculation significantly reduces the computation requirements without significantly affecting the quality of the weight calculation. However, if the interference signal is changing such as in the case of a pulsed interference signal, reducing the number of samples provided the adaptive weight calculator could result in non-recognition and thus non-cancellation of the interference signal. Using the above example to illustrate the point, the interference may only be present during the nine samples that were skipped, in which case the presence of the interference would not be sensed by the weight calculator and therefore would not be cancelled by the adaptive array.
Therefore, it would be advantageous to have an adaptive antenna array system that could identify both tone and pulse interference signals while still achieving the computational savings obtained by sparse sampling as described above in the example.
In accordance with one aspect of the present invention, a preprocessor is provided for use in an adaptive antenna array. The preprocessor reduces the sample rate of the signal prior to inputting the signal to the weight calculator (thereby reducing computation in the weight calculator), without xe2x80x9cmissingxe2x80x9d the pulsed interference signal as might occur with sparse sampling. The preprocessor is designed to achieve similar performance for other interference waveforms, including continuous wave (CW) tone interference. The preprocessor includes an input terminal for receiving an electromagnetic signal from an antenna element of the adaptive antenna array, and a frequency smearer operatively coupled to the input terminal. The frequency smearer is provided in order to smear the electromagnetic signal by varying a frequency of the electromagnetic signal across a predetermined frequency band and outputting the smeared electromagnetic signal to an averaging circuit. The averaging circuit, which is operatively coupled to the output of the frequency smearer, repetitively computes and outputs an average with respect to time of the smeared electromagnetic signal. Smearing reduces the possibility that a CW tone interference will be eliminated by the smoothing process (which would lead to an incorrect weight calculation that may prevent the adaptive array from nulling that interference).
In accordance with another aspect of the present invention, a preprocessor is provided in which a chirp waveform is applied to the electromagnetic signal in order to linearly vary the frequency of the electromagnetic signal across the predetermined frequency band. A variation on this approach is the use of other types of waveforms that provide the same effect, that of smearing the frequency content of the interference signals so that they are preserved in the averaging circuit.
In accordance with a further aspect of the present invention, a preprocessor is provided that includes means for sampling the smeared electromagnetic signal. The means for sampling creates a plurality of samples, which are provided to the averaging circuit in order to compute an average of a portion of the plurality of samples, thereby computing an average of that plurality of samples.
In accordance with another aspect of the present invention, an adaptive antenna array system is provided, which includes an array of antenna elements each for receiving an electromagnetic signal. The system further includes an input for receiving the electromagnetic signal from each of the plurality of antenna elements, and a frequency smearer operatively coupled to the input for smearing the electromagnetic signal by varying a frequency of the electromagnetic signal across a predetermined frequency band. The frequency smearer outputs the smeared electromagnetic signal via an output to an averaging circuit, which is operatively coupled to the output of the frequency smearer. The averaging circuit repetitively computes and outputs an average with respect to time of the smeared electromagnetic signal. The averaging circuit provides the average to an adaptive weight calculator, which calculates and outputs weighting coefficients based upon the average with respect to time of the smeared electromagnetic signal to a beam former that is also operatively coupled to the input in order to receive the electromagnetic signal from each of the plurality of antenna elements. The beam former serves to combine the electromagnetic signal from each of the plurality of antenna elements with the weighting coefficients to produce an output signal for the adaptive antenna array system.
In accordance with still another aspect of the present invention, a method of calculating weighting coefficients for an electromagnetic signal received by an adaptive antenna array is provided. The method includes smearing a frequency of the electromagnetic signal by varying the frequency across a predetermined frequency band, and sampling the smeared electromagnetic signal to create a plurality of samples thereof. The method further includes computing an average of a portion of the plurality of the samples to create an averaged sample, and, finally, using the averaged sample to calculate the weighting coefficients for the electromagnetic signal.